Cadmium selenide quantum dot (CdSe QD) nanocrystals emit visible light and have a quantum yield greater than many organic dyes. As a result these materials are great candidates for biomedical imaging and flow cytometry, but they contain toxic cadmium metal and are not soluble in water. Nontoxic water soluble QDs were created by the addition of the inert zinc sulfide (ZnS) shell and exchange of native hydrophobic ligands with hydrophilic water soluble ligands. In water the biocompatible QDs exhibit an enhanced emission and quantum yield when compared to the CdSe QD cores. An additional enhancement of the emission was seen with the use of ethylenediamine (EDTA) to facilitate the exchange of native oleic acid ligand for the water soluble 3-mercaptopropoinic acid (MPA) ligand. Unfortunately, it was found that the bright nontoxic QD materials are not stable over time in water and fall out of solution within days after ligand exchange. The stability of the biocompatible QDs, as measured by emission, were monitored for up to three weeks. In all cases the stability decreased over time with a greater decrease seen in the QDs that used EDTA in the ligand exchange. In order to use these materials in biomedical imaging, further work is needed to increase the stability of the materials in water while maintaining their high quantum yield.

Effect of ethylenediamine on the emission and stability of water soluble cadmium selenide zinc sulfide core shell quantum dot nanocrystals

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